1. Field of the Invention
The present invention relates to composite overwrapped pressure vessels (COPV""s) and their method of manufacture. More specifically, the present invention relates to high-performance COPV""s including liners made of metals which exhibit high moduli of elasticity and high ductility, such as titanium alloys.
2. General Background
The basic technology for composite overwrapped pressure vessels with metal liners dates back to the late 60""s and early 70""s.
High-performance fibers offer very high strength-to-weight ratios and are ideal for making lightweight pressure vessels. However, composite laminates fabricated with these fibers have relatively high permeability and cannot contain high pressure liquids or gasses or low pressure gasses for extended periods of time. Therefore, composite pressure vessels must have a liner to prevent leakage. The tank efficiency, as measured by its pressure multiplied by its volume divided by its weight (PV/W), increases as the liner weight decreases. For low pressure and/or liquid containment, elastomeric or polymeric liners are usedxe2x80x94these liners are strictly non-structural. For high pressure or gas containment, metal liners are typically used. Metal liners may be structural or non-structural.
For lightweight, high-pressure gas containment, there are basically two primary technologies (a) graphite/epoxy composite with a yielding aluminum liner, and (b) Kevlar/epoxy with load-sharing liners (typically stainless steel, titanium alloy, or inconel). The aluminum-lined, graphite/epoxy tank is the most prevalent technology, but it has limitations. First, the liner yields on each pressure cycle because the strain capability of the fibers is much higher than the elastic capability of the liner. This limits cycle life to around 100 cycles (depending on the specific design) and means that the liner is basically non-structuralxe2x80x94it adds weight and a permeation barrier but very little load-carrying capability. PV/W (burst pressure of the tank in p.s.i. times volume in cubic inches, divided by weight in pounds) for these tanks is typically about 1.0xc3x97106 in. These liners are typically spun from sheet metal or machined from forgings. The spun tanks typically have welded fittings at the ends of the tanks where the forged tanks typically are made in two halves and welded at the center.
The second type of tank makes use of a liner which has a higher elastic range and remains elastic during operating pressure cycles. FIG. 1 shows a comparison of liners which undergo plastic deformation each cycle (copper and aluminum) as compared to an elastic liner (titanium). This means the tank has the potential to be more efficient; however, because of the density of these materials and the thicknesses required for processing, the efficiency of these tanks is also about 1.0xc3x97106 in. These load-sharing lined tanks typically have much higher cycle life, but are also typically are much more expensive than aluminum-lined tanks because of the materials and manufacturing processes required, e.g. machining thick, expensive titanium forgings.
The present invention is a COPV with a high PV/W (preferably at least 1.05 million inches, more preferably at least 1.25 million inches, and even more preferably at least 1.45 million inches, more preferably at least 1.80 million inches, and most preferably at least 2.00 million inches). The present invention is able to achieve such a high PV/W in part because it uses a liner made of a high-strength metal which has a low modulus of elasticity and good ductility. The preferred metals are titanium alloys. More preferably, the metal is from the group consisting of titanium alloyed with Al, Cb, Cr, Fe, Mo, Si, Sn, Ta, V, and/or Zr. The most preferred material to use for the metal liner of the tank of the present invention is Ti-6Al-4V.
The apparatus of the present invention is a composite overwrapped pressure vessel, comprising a liner made of a metal having a tensile yield strength in p.s.i. divided by tensile modulus of elasticity in p.s.i. (FTY/E) of preferably at least 0.6% and having a ductility of preferably at least 5%, the liner including first and second dome portions and a cylinder portion, and a composite overwrap applied over the liner, wherein the vessel has a PV/W of at least 1.25 million inches. The metal is preferably a titanium alloy from the group consisting of: Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-2.5Sn, Ti-5Al-2.5Sn ELI, Ti-6Al-2Cb-1Ta-0.8Mo, Ti-8Al-1Mo-1V, Ti-11Sn-5Zr-2Al-1Mo, Ti-6Al-4V, Ti-6Al-4V ELI, Ti-6V-2Sn, Ti-3Al-2.5V, Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-13V-11Cr-3Al, Ti-3Al-8V-6Cr-4Mo4Zr, Ti-15V-3Al-3Cr-3Sn, and Ti-10V-2Fe-3Al. The term xe2x80x9cELIxe2x80x9d stands for xe2x80x9cextra low interstitialxe2x80x9d.
More preferably, the metal has a FTY/E of at least 0.7%, even more preferably at least 0.8%, even more preferably at least 0.9%, and most preferably at least 1.0%.
The ductility is more preferably at least 10%, even more preferably at least 15%, even more preferably at least 20%, even more preferably at least 25%, and most preferably at least 30%.
While other types of welds might work, the welding steps used to make the liner are preferably done with an autologous fusion process. More preferably, the welding process is electron beam welding, and most preferably, pulsed electron beam welding.
There is preferably an adhesive between the liner and the overwrap, and the adhesive is preferably a film adhesive. Preferably, the COPV includes a protective coating over the overwrap.
Preferably, the liner of the COPV of the present invention has a ratio of thickness in inches over diameter in inches of about 1.7xc3x9710xe2x88x923. Preferably, the liner of the COPV has a thickness of not more than 0.050xe2x80x3, more preferably, not more than 0.040xe2x80x3, and most preferably not more than 0.025xe2x80x3. The ratio of the length of the cylinder to the diameter of the cylinder is preferably at least 1.00, more preferably at least 1.25, and more preferably greater than 1.25.
The overwrap can comprise a graphite/epoxy composite.
The present invention also comprises a method of manufacturing a composite overwrapped pressure vessel. The method preferably comprises the following steps:
(a) using spin forming, making a liner having first and second dome portions and a cylindrical portion made of a metal having a FTY/E of at least 0.6% and a ductility of at least 5%;
(b) forming first and second bosses made of the metal, the first boss being connected to the first dome portion and the second boss being connected to the second dome portion; and
(c) applying a composite overwrap over the liner, applying filaments of the overwrap onto the liner.
Welding steps are preferably done with an electron beam weld process.
It is an object of the present invention to produce a COPV which, when used in spacecraft, launch vehicles, or aircraft, effects significant savings as compared to current COPV""s.
It is an object of the present invention to produce a COPV with a high PV/W.
It is also an object of the present invention to produce a COPV with a liner made of a metal having a high FTY/E and a high ductility.
It is another object of the present invention to provide a method of producing a COPV with a high PV/W.
As used herein, including in the claims, PV/W stands for tank burst pressure in p.s.i. times volume of the tank in cubic inches, divided by the weight of the tank in pounds. PV/W is expressed in inches.